Fuel cells use energy generated in the binding reaction between hydrogen and oxygen to generate electric power. From this nature, fuel cells are expected to be introduced and popularized in terms of both energy saving and environmental friendliness. The types of the fuel cells include solid electrolyte, molten carbonate, phosphoric acid, proton exchange membrane, and other types.
Of these types, proton exchange membrane fuel cells have high power density and provide an opportunity to downsize. In addition, the proton exchange membrane fuel cells operate at temperatures lower than those at which other types of fuel cells operate, and are easy to start up and stop. With such advantages, the proton exchange membrane fuel cells are expected to be used for automobiles and small-sized cogeneration for home use and have particularly received attention in recent years.
FIG. 1A is a perspective view of a proton exchange membrane fuel cell (hereinafter, simply also referred to as a “fuel cell”), illustrating the entire fuel cell made up of a combination of a plurality of single cells. FIG. 1B is an exploded perspective view of a single cell constituting the fuel cell.
As illustrated in FIG. 1A, a fuel cell 1 is a set (stack) of single cells. In each single cell, as illustrated in FIG. 1B, on one surface of a proton exchange membrane 2, an anode-side gas diffusion electrode layer (also called a “fuel electrode film”; hereinafter, referred to as an “anode”) 3 is arranged. On the other surface of the proton exchange membrane 2, a cathode-side gas diffusion electrode layer (also called an “oxidant electrode film”; hereinafter, referred to as a “cathode”) 4 is arranged. On both surfaces of the stacked body, separators (bipolar plates) 5a and 5b are arranged.
Examples of the fuel cells include a water-cooled fuel cell in which a separator having a distribution channel for cooling water is arranged between two adjacent single cells or for several single cells. The present invention also covers a titanium product for a separator of such a water-cooled fuel cell.
As the proton exchange membrane (hereinafter, simply referred to as an “electrolyte membrane”) 2, a fluorine-based proton conducting membrane having a hydrogen-ion (proton) exchange group is mainly used.
The anode 3 and the cathode 4 are each made up principally of a carbon sheet that is consisted of carbon fibers having good conductivity into a sheet shape (or a sheet of carbon paper thinner than the carbon sheet, or a piece of carbon cloth still thinner than the carbon sheet). The anode 3 and the cathode 4 are each provided with a catalyst layer in some cases. The catalyst layer consists of a particulate platinum catalyst, graphite powder, and as necessary, a fluororesin having a hydrogen-ion (proton) exchange group. In this case, this catalyst layer comes into contact with a fuel gas or an oxidative gas to promote the reaction.
On a surface of the separator 5a on the anode 3 side, groove-shaped channels 6a are formed. Through these channels 6a, a fuel gas (hydrogen or a hydrogen-contained gas) A is caused to flow, so as to supply hydrogen to the anode 3. On a surface of the separator 5b on the cathode 4 side, groove-shaped channels 6b are formed. Through these channels 6b, an oxidative gas B such as air is caused to flow, so as to supply oxygen to the cathode 4. The supply of these gasses causes an electrochemical reaction to generate DC power.
The main functions required for separators of proton exchange membrane fuel cells are as follows.    (1) The function as a “channel” for uniformly supplying a fuel gas or an oxidative gas to the inside of a cell surface.    (2) The function as a “channel” for efficiently discharging water generated on the cathode side from the fuel cell out of the system, together with carrier gasses such as air after the reaction, and oxygen.    (3) The function of serving as a path of electricity by being in contact with an electrode film (the anode 3, cathode 4), and further serving as an electrical “connector” between two adjacent single cells.    (4) The function as a “partition wall” between adjacent cells, between an anode chamber of one cell and a cathode chamber of the adjacent cell.    (5) In a water-cooled fuel cell, the function as a “partition wall” between a cooling water channel and an adjacent cell.
The substrate material of a separator used for a proton exchange membrane fuel cell (hereinafter, simply referred to as a “separator”) needs to be one that can fulfill such functions. The substrate material is roughly categorized into a metal-based material and a carbon-based material.
A separator consisting of a carbon-based material is produced by the following method, for example.                Method in which a graphite substrate is impregnated with a thermosetting resin such as a phenol-based thermosetting resin and a furan-based thermosetting resin for hardening, and baked.        Method in which carbon powder is mixed with a phenolic resin, a furan resin, a tar pitch, or the like, subjected to press molding or mold injection into a plate shape, and baked to be a glassy carbon.        
Using a carbon-based material has an advantage of obtaining a lightweight separator, but involves a problem of having gas permeability, and a problem of a low mechanical strength.
As the metal-based material, titanium, stainless steel, carbon steel, or the like is used. A separator consisting of one of these metal-based materials is produced by press working or the like. The metal-based material is excellent in formability as an intrinsic characteristic of metals. This allows the reduction of the thickness of a separator, so as to achieve weight reduction of the separator.
However, the conductivity of the surface of a separator consisting of a metal-based material may decrease over time. Such a decrease in conductivity occurs due to the oxidation of the surface of the separator. In addition, under an environment containing fluorine (e.g., an environment in which fluorine is supplied from an electrolyte membrane containing fluorine), the conductivity of the surface of the separator also decreases by a fluoride generated due to corrosion of the surface of the separator. This causes a problem of a possible increase in contact resistance between a separator consisting of a metal-based material and gas diffusion layers (an anode and a cathode). To solve this problem, the following measures are proposed.
Patent Document 1 proposes that, in a titanium separator substrate, a passivation film is removed from a surface to come into contact with an electrode, and thereafter the surface is plated with a noble metal such as gold. However, using a noble metal in a large quantity raises a problem from the viewpoints of economic efficiency and restriction of resources. The proton exchange membrane fuel cells are expected to be widely used as fuel cells for mobile objects and stationary fuel cells. Employing the method of Patent Document 1 for producing separators for proton exchange membrane fuel cells results in use of a noble metal in a large quantity. For this reason, the method of Patent Document 1 does not come into widespread use.
Patent Document 2 proposes a titanium alloy in which a rise in contact resistance is suppressed by pickling a titanium alloy that contains one, or two or more kinds of platinum group elements to concentrate the platinum group elements on the surface of the titanium alloy. Patent Document 3 proposes a titanium separator in which a platinum group element is concentrated on the surface of the separator by pickling, and the surface is thereafter subjected to heat treatment under a low-oxygen-concentration atmosphere for the purpose of improving the adhesiveness between the platinum group element concentrated on the surface and a matrix. However, both separators contain platinum group elements and require many steps in production, and thus a significant rise in cost is inevitable.
For this reason, Patent Document 4 describes the attempt to solve the problems described above without using a noble metal. Specifically, a method is proposed in which a conductive contact layer consisting of carbon is formed on a titanium surface of a metallic separator by vapor deposition on the surface.
Patent Document 5 proposes a method in which a conductive ceramic is dispersed on a separator surface to reduce a contact resistance.
Patent Document 6 discloses a titanium plate material that is formed with a titanium substrate layer and a surface layer. The surface layer includes a titanium layer in which a compound is intermixed with Ti (metallic titanium) containing O (oxygen), C (carbon), and N (nitrogen) dissolved. The formed compound includes Ti and one or more kinds of O, C, and N. Patent Document 6 describes that the presence of the titanium layer under an outermost layer or a passivation film of the titanium plate material provides a surface with a reduced contact resistance.
Patent Document 7 discloses a separator material for fuel cells in which an oxidized layer is formed between a Ti substrate and a Au layer or a Au alloy layer at a thickness of 5 to 30 nm, the oxidized layer containing 20% by mass or more of O.